1. Field of the Invention
The present invention relates to an optical system and an optical apparatus having the optical system.
2. Description of the Related Art
In general, in the optical system used in optical apparatuses including digital cameras, video cameras, and copying machines, as the miniaturization of the entire optical system is extended by reducing the entire lens length (the entire length of the optical system, which is the distance between the first lens surface adjacent to the object and the image plane), the more chromatic aberration including axial chromatic aberration and transverse chromatic aberration (chromatic aberration of magnification) is generated so as to have a tendency to reduce optical performances.
In particular, in a Gauss-type optical system and a modified Gauss-type optical system, such as a Xenoter type and an ortho-meter type, in that lenses are arranged roughly symmetrically with respect to an aperture diaphragm, since an F number is comparatively small (bright) as well as an angle of view is comparatively large, it has been difficult to correct chromatic aberration over the entire picture while suppressing various aberrations including spherical aberration, coma, and astigmatism.
In order to reduce such chromatic aberration, methods using an extraordinary partial dispersion material and using a diffractive optical element in an optical path are generally well-known.
As a method for correcting the axial chromatic aberration over the broad wavelength band, an image-read lens with the chromatic aberration corrected using an optical material, such as fluorite, having low refractive and dispersion functions and extraordinary partial dispersion has been known (Japanese Patent Laid-Open No. 2001-337271, corresponding to U.S. Pat. No. 6,437,926).
Since the extraordinary partial dispersion material of the optical material such as fluorite has a low refraction index of about 1.5, the material can be used in an optical system with a focal length longer than that of a medium-telephoto lens and with the comparatively small change in aberration due to the change in angle of view. However, it can be difficult to correct various aberrations in a well-balanced state using the extraordinary partial dispersion material in optical systems other than this optical system.
Also, a Gauss-type optical system has been known that can use a diffractive optical element so as to correct and/or reduce the chromatic aberration without using the extraordinary partial dispersion optical material (Japanese Patent Laid-Open No. H10-170818, corresponding to U.S. Pat. No. 5,930,043).
Also, a liquid material having characteristics of comparatively high dispersion and extraordinary partial dispersion is known among materials, having chromatic-aberration correction functions, related to the diffractive optical element, and an achromatic optical system using the liquid material is proposed (U.S. Pat. Nos. 5,731,907 and 5,638,215).
When a lens made of a material, such as fluorite, with low refraction and low dispersion is used in the Gauss-type optical system and the modified Gauss-type optical system, such as a Xenoter type and an ortho-meter type, frequently employed for a standard lens of a single-lens reflex camera for 35 mm film, the change in angle of view of aberration is large because of the large angle of view, and furthermore, the chromatic aberration does not change larger than a predetermined value if the refracting power of the lens surface is not largely changed. Therefore, it can be difficult to correct both the chromatic aberration and the various aberrations such as the spherical aberration, the coma, and the astigmatism.
On the other hand, although the diffractive optical element has a sufficient correction function of the chromatic aberration, unnecessary diffraction light in the order of diffraction other than that designed for practical use is generated.
Since the material discussed in U.S. Pat. Nos. 5,731,907 and No. 5,638,215 is liquid, when it is used as an optical material, the manufacturing is difficult because of the necessity of the structure for sealing it. Also, characteristics, such as the refracting index and the dispersion, are largely changed with the change in temperature, so that the environmental resistance is insufficient. Furthermore, the interface to air is not obtained, so that the chromatic aberration is difficult to be sufficiently corrected.